1. Field of the Invention
The present invention relates to a digital camera, and particularly to a digital camera equipped at least with a high-speed electronic shutter.
2. Description of the Related Art
In recent years, digital cameras which take still pictures or moving pictures of image subjects by utilizing a solid state imaging system, whose structure includes a solid-state imaging element such as a CCD (charge coupled device) sensor or the like for photographing a subject, have become common.
“Interline transfer-type CCD sensors” (referred to simply as “CCD sensors” below) are often employed in such digital cameras. An interline transfer-type CCD sensor is provided with photodiode (PD) sections, a plurality of vertical transfer registers, a horizontal transfer register, transfer gates (TG), and an overflow drain (OFD). The PD sections serve as solid-state imaging elements, which are, for example, rectangularly disposed at respective pixels, and which photoelectrically convert received light into electronic signals. The vertical transfer registers read out and transfer charges that have accumulated at the PD sections. The transfer gates are for transferring charges from the PD sections to the vertical transfer registers. The horizontal transfer register transfers charges that have been transferred from the vertical transfer registers. The overflow drain is for discharging unneeded charges that remain at the PD sections.
At the time of an operation for receiving light in accordance with a subject image, this CCD sensor operates the OFD for achieving an electronic shutter function, completely flushes charge remaining at the PD sections, and then closes the TGs and enters a state in which charge accumulation is possible (a light-reception capable state). In this light-reception capable state, light in accordance with the subject image is received for a predetermined exposure duration (a charge accumulation duration), and charges are accumulated at the PD sections. Then, at the time of a charge transfer operation, which transfers and outputs the charges accumulated at the pixels by the light-receiving operation, the TGs are opened and the charges accumulated at the PD sections are transferred to the vertical transfer registers. The charges transferred to the vertical transfer registers are serially transferred to the horizontal transfer register, and then outputted from the horizontal transfer register. Thus, the charges accumulated at the respective PD sections are read out.
For a method for reading out the accumulated charges at this time, a field accumulation method and a frame accumulation method are commonly utilized.
The frame accumulation method is a method in which a single frame is divided into two fields (an odd field and an even field). Charges accumulated at light-receiving elements of odd lines, which are in the odd field, and charges accumulated at light-receiving elements of even lines, which are in the even field, are transferred and read out alternatingly. In this method, at a field shift time for the odd field, the charges accumulated at pixels of the odd lines are transferred line-by-line in the vertical direction at the vertical transfer registers, and then line shifting is carried out and the accumulated charges of the pixels of the odd lines are serially read out from an output terminal. In the meantime, accumulation of charges is being carried out at the pixels of the even lines. Thereafter, the charges accumulated at the pixels of the even lines in the even field are transferred to the vertical transfer registers, line shifting is carried out, and the charges accumulated at the pixels of the even lines are serially read out from the output terminal.
In contrast, the field accumulation method is a method which reads out by, for an odd field, melding and transferring charges accumulated at pixels of a line that is an odd number from a horizontal transfer register side and of a next even-numbered line and, for an even field, melding and transferring charges accumulated at pixels of a line that is an even number from the horizontal transfer register side and a next odd-numbered line.
That is, in the frame accumulation method, accumulated charges are always read out for only half of all pixels in each field, but in the field accumulation method, reading out is performed for charges accumulated at all pixels in both fields.
When charges are read out by the field accumulation method, in order to suitably control potentials of the element main body, two kinds of transfer gate pulse, which have different occurrence timings, are required for the two fields. In contrast, when charges are read out by the frame accumulation method, only one type of transfer gate pulse is needed.
In these interline-type CCD sensors, in a case in which a shutter speed of the electronic shutter is fast, a difference between exposure times of the odd field and the even field occurs at the time of reading out the accumulated charges, which leads to image deterioration, such as the occurrence of flicker and the like.
In order to deal with this problem, a technique has been proposed in, for example, Japanese Patent Application Laid-Open No. 6-133226. In this technique, exposure times of the fields are made equal by adjusting output timings of shutter pulses for the electronic shutter.
However, in an all-pixel-readout type CCD, such as a honeycomb CCD proposed by the applicant of the present invention, it is difficult to apply the prior art technique mentioned above so as to provide a structure in which the electronic shutter resets the charges in all photo diodes (PD) at the same time.
Moreover, with the aforementioned honeycomb CCD, or a conventional interline-type CCD in which the light-receiving elements are rectangularly disposed, in processing for reading out moving images, readout timings (timings for transfer of the charges from the PDs to the vertical transfer registers) are different for different colors. Consequently, particularly when using an electronic shutter with a high shutter speed (for example, 1/10,000 second, 1/100,000 second, or the like), differences in exposure times lead to gain errors. Consequently, obvious color shifting may occur, and image deterioration may result (see FIG. 6).